For blow moulding applications, PE resins are required in general to combine good processa-bility, high surface quality of the finished article and a good balance of mechanical properties (stiffness, impact resistance, environmental stress crack resistance). Already this is difficult to achieve simultaneously for Ziegler products. Special applications require the polyethylene to fulfil additional properties.
Polymer compositions produced with Ziegler catalysts can be obtained by polymerizations in a series of two or more reactors in which the polymerization is performed under different conditions so that it results at the end in a multimodal polymer composition comprising different polymer fractions with different molecular weight, different molecular weight distribution and different density. Such multimodal polymer compositions can reveal polymer properties which cannot be achieved with a polymerization under constant single polymerization conditions. The manufacture of such multimodal polymer compositions for use in blow moulding of cans and containers obtained by Ziegler catalysis is described e.g. in EP-1228101. Use of a Ziegler catalyst ensures good processing properties of the ensuing polymer, and allows of obtaining good mechanical properties, in particular a good Environmental Stress Cracking Resistance (ESCR). Especially in industry, barrels and containers made from polyethylene materials are often used for packaging and transport of chemicals or other hazardous substances, such PE materials excelling by superior stiffness and stress crack resistance of the respective blow mouldings. When used for intermediate or large containers it is further important that the moulded article shows a good dimensional stability.
Polymer compositions suitable for blow moulding of large containers are often produced with Phillips-type chromium oxide catalysts which are known to display very little warpage, but suffers from very broad molecular weight distribution unsuitable for multimodal product engineering. The so produced polymer compositions show a typical gap swelling behaviour during the blow moulding process which is usually different from polymer compositions produced with Ziegler catalysts, the latter having usually a lower gap swell if compared with chromium catalyst products.
In WO2004/058878 a polyethylene composition is described which is produced in a series of three reactors with a Ziegler catalyst. Such polyethylene composition shows advantageous properties, however, the composition shows during the blow moulding process a swelling be-haviour different from the often applied chromium catalyst polymer products. Such different swelling behaviour can cause problems for the blow moulding operation because the machin-ery may need adjustments to the deviating swelling behaviour.